JPH07197186A - Hot-rolled steel sheet having >=980n/mm2 strength represented by excellent delayed fracture resistant characteristic and its production - Google Patents

Abstract

PURPOSE:To produce a hot-rolled steel sheet having high strength represented by an excellent delayed fracture resistant characteristic and to provide a produc tion method therefor. CONSTITUTION:This steel sheet is the hot-rolled steel sheet consisting of 0.10-0.35% C, <=1.5% Si, 1.0-3.5% Mn, <=0.03% P, <=0.02% S, 0.02-0.10% Al, <=0.01% N, and further containing one or more kinds among 0.04-0.20% Ti, <=0.10% V, <=0.10% Nb, and as necessary, further one or more kinds among <=1.5% Mo, <=0.01% B, <=1.5% Cr and the balance iron with inevitable impurities and having a structure mainly containing fine and low temp. transformation producing material and >=980N/mm<2> strength represented by the excellent delayed fracture resistant characteristic. The steel sheet is produced by executing cooling at >=10 deg.C/s cooling rate to the steel having this composition after finishing the hot-rolling in the temp. range of >=Ar3, and coiling at <=450 deg.C. The steel sheet is suitable for a bumper, a reinforcing member for automotive door, etc.

Description

Detailed Description of the Invention

[0001]

The present invention is suitable for bumpers of automobiles, reinforcing members for doors, etc., and has a strength of 980 N / mm ² or more (particularly 1
The present invention relates to an ultrahigh strength hot rolled steel sheet having 180 N / mm ² or more) and excellent delayed fracture resistance and a method for producing the same.

[0002]

^2. Description of the Related Art There is a strong demand for improving fuel efficiency of automobiles due to global environmental problems, etc. Therefore, in order to reduce the weight of the vehicle body, the bumper, the reinforcing member of the door, etc., have a super high of 980 N / mm ² or more. There is an increasing need for high strength steel sheets, and there are JP-A-2-197525 and JP-A-5-594.
As can be seen in No. 93 etc., many studies have been made on increasing the strength of ultra-high strength hot-rolled steel sheets.

However, a bolt using an ultra-high strength steel having a strength of 980 N / mm ² or more may cause cracking due to hydrogen embrittlement, as disclosed in, for example, JP-A-60-15564.
It is known that, even in ultra-high strength thin steel sheets, hydrogen generated by the corrosion reaction in the atmospheric environment enters the steel sheets and is suddenly destroyed during use. Almost no studies have been found on the improvement of delayed fracture resistance of thin steel sheets.

Under these circumstances, the present invention solves the above-mentioned problem of delayed fracture in an ultra-high strength hot-rolled steel sheet having a tensile strength of 980 N / mm ² or more and has a good delayed fracture resistance. It is an object of the present invention to provide an ultra high strength hot rolled steel sheet and a method for manufacturing the same.

[0005]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have investigated the relationship between strength and delayed fracture in cold rolled materials and hot rolled materials containing Ti. As shown in Table 1 and FIG. 1, when the hot rolled material that is normally wound at a temperature of about 400 ° C. is hot-rolled, water-cooled and then rolled at room temperature, We have found an interesting fact that delayed fracture resistance is better than that. It is conventionally known that Ti is added to improve the fatigue property, but it is a new property that the delayed fracture resistance can be remarkably improved by adding Ti in an ultra-high strength steel plate and in a hot rolled steel plate. The present invention has been completed through further detailed experimental research based on this finding.

[0006]

[Table 1]

That is, according to the present invention, C: 0.1 to 0.3.
5%, Si ≦ 1.5%, Mn: 1.0-3.5%, P ≦ 0.0
3%, S ≦ 0.02%, Al: 0.02 to 0.10%, N ≦
0.01%, and further 0.04% ≦ Ti ≦ 0.2
0%, V ≤ 0.10%, Nb ≤ 0.10%, and one or more of them, and if necessary, Mo ≤ 1.5%, B
≤0.01%, Cr ≤1.5%, 1 type or 2 types or more, the balance consisting of iron and unavoidable impurity elements, and having a structure mainly composed of fine low-temperature transformation products. The gist is a hot rolled steel sheet having a strength of 980 N / mm ² or more, which is excellent in delayed fracture resistance.

Another aspect of the present invention is C: 0.1 to 0.3.
5%, Si ≦ 1.5%, Mn: 1.0-3.5%, P ≦ 0.0
3%, S ≦ 0.02%, Al: 0.02 to 0.10%, N ≦
0.01%, Ti ≦ 0.20%, V ≦
One or more of 0.10% and Nb ≦ 0.10% are included, and if necessary, Mo ≦ 1.5%, B ≦ 0.01.
%, Cr ≦ 1.5%, one or two or more, and the balance having a chemical composition of iron and inevitable impurity elements, after hot rolling in a temperature range of Ar ₃ or more, 1
980 N / excellent delayed fracture resistance, characterized by cooling at a cooling rate of 0 ° C / s or more and winding at 450 ° C or less
The gist is a method for manufacturing a hot-rolled steel sheet having a strength of mm ² or more.

[0009]

The present invention will be described in more detail below. First, the reasons for limiting the chemical composition of steel in the present invention will be described.

C: C is an essential element for forming a low-temperature transformation product and increasing the strength, and 0.10% or more is necessary to obtain a strength of 980 N / mm ² or more. But 0.3
If it exceeds 5%, the delayed fracture resistance deteriorates, so the C content is made 0.10 to 0.35%.

Si: Si is an element effective for increasing the strength without deteriorating the delayed fracture resistance. However, if added in excess of 1.5%, the paintability is deteriorated, so the Si content is 1.5%.
Below.

Mn: Mn is an element effective in increasing the strength by forming a low temperature transformation product by improving the hardenability, and is 1.0% or more to obtain a strength of 980 N / mm ² or more. is necessary. However, if it exceeds 3.5%, the delayed fracture resistance deteriorates, so the Mn content is made 1.0 to 3.5%.

P: P segregates at grain boundaries and deteriorates brittleness, so is suppressed to 0.03% or less.

S: S forms an inclusion and becomes a hydrogen accumulation site, deteriorating the delayed fracture resistance, so it is suppressed to 0.02% or less.

N: N is a solid solution in steel and deteriorates press workability, so the content is made 0.01% or less.

Al: Al is added in an amount of 0.02% or more for deoxidation, but since the surface properties are deteriorated, its upper limit is 0.02.
10%.

One, two or more of Ti, Nb and V: Ti is an element which is effective not only for increasing the strength but also for improving the delayed fracture resistance. This is considered to be because, by suppressing the recrystallization and transformation of the processed austenite rolled during hot rolling, the structure becomes finer, the grain boundaries are less likely to break, and brittleness is improved. However, 0.20%
Since the effect is saturated when it exceeds, the Ti content is set to 0.20% or less. In addition, since Ti easily forms an oxide during welding and the weldability may be deteriorated, it is preferable that the addition amount be small, especially for applications where welding specifications are strict, but the above effect is 0.
Since it is remarkable at 04% or more, 0.04% or more is preferable.

Further, Nb and V are effective not only for increasing the strength but also for refining the structure, so that they are also effective for improving the delayed fracture resistance. However, the effect is saturated even if added in excess of 0.10%, so this is the upper limit.

Although Ti, Nb, and V are effective when added alone, they can be added together, and since Ti addition is more effective than other components, Ti addition or a combination of Ti and other components is effective. Addition is desirable.

Mo, B, and Cr: Mo, B, and Cr are elements that are effective in increasing the strength because they form a low-temperature transformation product by improving hardenability without deteriorating delayed fracture resistance. If necessary, one kind or two or more kinds may be added. When added, Mo is 1.5% and B is 0.1%.
Since the effect is saturated at 01% and Cr at 1.5%, the upper limits are set respectively. Since Mo is an expensive element, it is desirable to set it to 0.8% or less.

It should be noted that the effect of the present invention is not impaired even if elements such as Cu and Ni are added within the range of 1.5% or less for further strengthening.

Next, the manufacturing conditions of the present invention will be described.

Steel having the above chemical composition is melted and cast by a conventional method, hot-rolled after heating, but when hot-rolling finish temperature is mixed with work ferrite in the two-phase region, workability is deteriorated, resulting in automobile parts. Since it becomes difficult to process into etc., Ar ₃ or more is selected.

The cooling rate and winding temperature after hot rolling are
980 N / mm ² or more strength, especially since to obtain a 1100 N / mm ² or more intensity is needed tissue consisting mainly of low-temperature transformation product, the cooling rate 10 ° C. / min or more, the coiling temperature 450 ° C. Below (including room temperature).

Under these manufacturing conditions, a structure mainly composed of fine low-temperature transformation products can be obtained. Low-temperature transformation products include martensite, bainite, etc., which do not substantially contain ferrite.

It should be noted that, after the winding, a tempering treatment at a temperature of 450 ° C. or lower for adjusting strength and workability or a skin pass treatment for shape correction does not impair the effects of the present invention.

Next, examples of the present invention will be described.

[0028]

Example 1 An ingot having the chemical composition shown in Table 2 was heated to 1230 ° C., hot-rolled to a plate thickness of 2.4 mm under the conditions shown in Table 1, and showered to a predetermined temperature from 550 ° C. to room temperature. After cooling, it was wound up. Then, the surface was ground to 1.8 mm and the strength and delayed fracture resistance were investigated. The results are shown in Table 2.
Shown in.

The delayed fracture resistance is 15 mm × 65
Using a potentiostat in a 0.5 mm / liter sulfuric acid + 0.01 mol / liter KSCN solution, a strip test piece of mm loaded with a bending stress of 980 N / mm ² is less noble than the self-potential of −800 mV. An electric potential was applied, and the time when cracking occurred was evaluated.

As is clear from Table 2, all of the examples of the present invention show high strength of 980 N / mm ² or more and good delayed fracture resistance (crack generation time ≧ 10 min). On the other hand, Comparative Examples No. 15 and 17 do not have the desired strength.
Also, in Comparative Examples Nos. 16, 18, and 19, the crack initiation time is 1
It is 0 min or less, and the delayed fracture resistance is inferior to the examples of the present invention.

[0031]

[Table 2]

[0032]

Example 2 12 ingots of the chemical composition shown in Tables 3 and 4 were used.
It was heated to 30 ° C., hot-rolled to a plate thickness of 2.4 mm under the conditions shown in Table 3, cooled to a predetermined temperature from 550 ° C. to room temperature at various cooling rates, and wound. Then, the surface was ground to 1.8 mm and the strength and delayed fracture resistance were investigated. The results are shown in Tables 3 and 4. The evaluation method of delayed fracture resistance is the same as in Example 1.

As is clear from Tables 3 and 4, all of the examples of the present invention exhibit high strength of 980 N / mm ² or more and good delayed fracture resistance (crack generation time ≧ 10 min).
On the other hand, Comparative Examples Nos. 4, 7, 12, 14, 17, 19,
23, 24 and 26 do not have the desired strength. In addition, in Comparative Examples Nos. 25, 27 and 28, the crack initiation time is 10 mi.
It is n or less and the delayed fracture resistance is inferior to the examples of the present invention.

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

As described in detail above, according to the present invention,
Ultra-high-strength hot-rolled steel sheets with good delayed fracture resistance and tensile strength of 980 N / mm ² or more can be easily obtained, and are particularly suitable for automobile bumpers, door reinforcing members and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between delayed fracture resistance (cathode charge cracking occurrence time) and strength (TS) for hot rolled materials and cold rolled materials.

Claims (3)

[Claims] 1. By weight%, C: 0.10 to 0.35%, Si ≤ 1.5%, Mn: 1.0 to 3.5%, P ≤ 0.03%, S ≤ 0.02. %, Al: 0.02 to 0.10%, N ≦ 0.01%, and further 0.04% ≦ Ti ≦
1 out of 0.20%, V ≦ 0.10%, Nb ≦ 0.10%
1 or 2 or more, the balance being iron and unavoidable impurity elements, and having a structure mainly composed of fine low-temperature transformation products, which is excellent in delayed fracture resistance 9
A hot rolled steel sheet having a strength of 80 N / mm ² or more. 2. Further, Mo ≦ 1.5%, B ≦ 0.01%, Cr
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains ≤1.5% of one type or two or more types. 3. In% by weight, C: 0.10 to 0.35%, Si ≤ 1.5%, Mn: 1.0 to 3.5%, P ≤ 0.03%, S ≤ 0.02. %, Al: 0.02 to 0.10%, N ≦ 0.01%, and Ti ≦ 0.20%,
It contains one or more of V ≦ 0.10% and Nb ≦ 0.10%, and if necessary, Mo ≦ 1.5%, B ≦ 0.1.
After finishing hot rolling in a temperature range of Ar ₃ or higher for a steel containing 01% and Cr ≦ 1.5%, one or more kinds, and the balance having a chemical composition of iron and unavoidable impurity elements. 98 with excellent delayed fracture resistance characterized by being cooled at a cooling rate of 10 ° C / s or more and wound at 450 ° C or less
A method for manufacturing a hot-rolled steel sheet having a strength of 0 N / mm ² or more.